This invention relates to a method of forming a stable electrospray of a volatile liquid in a low-pressure environment while avoiding the tendency for the volatile liquid to freeze, boil, or evaporate.
There have been many efforts to produce stable electrosprays of liquids in a low-pressure environment, especially for use in electrical propulsion. In this electrospray technique, often referred to as colloidal propulsion, a conducting liquid is slowly injected through an electrified capillary tube. When the electrical potential between the liquid and its surroundings rises to a few kilovolts, the meniscus at the tube exit develops a conical shape, commonly referred to as the Taylor cone. A thin microthread of liquid is issued from the tip of the Taylor cone, which eventually fragments to form a spray of highly charged droplets.
Glycerol has traditionally been the propellant of choice in colloidal propulsion. However, the high viscosity and low electrical conductivity of glycerol have precluded the ability to produce the small charge drops desired and have led researchers to consider other propellant choices. Newer approaches have relied on the use of electrolytes based on formamide or other amides, glycols, organic phosphates and carbonates, certain molten salts, etc., which are much less viscous and far more conductive than those based on glycerol. These more favorable properties make it possible to produce charged drops in the diameter range of a few tens of nanometers, rather than the few hundred nanometers afforded by glycerol colloids, which in turn allows higher specific impulses at smaller acceleration voltages. Ideally, from the electrical propulsion viewpoint, it would be desirable for a variety of applications, to produce even smaller drops, since their charge over mass ratio would be further increased over the values now possible with formamide. However, this objective is precluded in formamide electrolytes by two obstacles: 1) electrical conductivities in room temperature formamide are limited to about 2 S/m; and 2) ions begin to evaporate from the meniscus surface at electrical conductivities of about 1.5 S/m, and the mixed emissions of drops and ions reduces considerably the propulsion efficiency.
The current situation as it relates to available fuels for electrical propulsion may be summarized with reference to FIG. 1, which demonstrates the range of mass over charge ratios attainable in principle via Taylor cones in a low-pressure environment. M/q is represented as m/z in the atomic mass units generally used in mass spectrometry (m/z xcx9c1 for H+). The gap below 105 Dalton may currently be covered only at limited propulsion efficiency in the mixed regime, where both ions and drops are produced.
On the right side of FIG. 1, one sees the range of m/z available from formamide and glycerol based colloidal sources. On the left side of FIG. 1, are ion sources based on gas sources (Xe), liquid metal ion sources (Cs+, Au+) and room-temperature molten salts (ionic liquids). The latter type includes existing ionic liquids whose masses extend almost to 1000 Dalton, as well as heavier ionic liquids that may be synthesized in the near future. No experiments have yet been carried out with ionic liquids other than with a few salts of 1-Ethyl-3-Methyl imidazolium+(EMIm+; m/z=111.2), so the ionic liquid bars in FIG. 1 remain hypothetical.
The significance of FIG. 1 follows from the fact that one of the major parameters available to optimize the propulsion system ideal for a particular mission is precisely m/z (the same holds for almost all applications of ion or charged particle beams). The considerations involved for electrical propulsion are complex and address primarily the energy required to accelerate the ejected fuel, as well as the impulse derived per unit mass of fuel. Light ions produce the highest specific impulse, but tend to deliver very small currents and at a high energy cost. The opposite limit is that of heavy charged particles. The optimal m/z is conventionally placed in the middle of the gap region shown (though this is mission and materials dependent). For that reason, one goal of the research on colloidal and ionic propulsion is aimed at identifying new materials able to fill various regions of that gap, including formamide and ionic liquids.
Patent Application Publication No. US 2002/0109104 A1, the disclosure of which is herein incorporated by reference in its entirety, describes a method of producing ions and nanodrops from Taylor cones at reduced pressures. This invention, however, is at present limited to a few liquids enjoying simultaneously the special properties of having low volatilities and high electrical conductivities. For many of the applications described in the above referenced Patent Application Publication, it would be highly advantageous to also be able to use more volatile liquids.
US 2002/0109104 A1 lists a number of materials suitable for forming Taylor cones in a vacuum. Some of these materials, such as formamide, do indeed produce Taylor cones in a vacuum. However, under the conditions of most interest for the US 2002/0109104 Application, formamide solutions are in fact sufficiently volatile to disrupt the operation of Taylor cones.
Gamero-Castaxc3x1o et al., Electrospray as a Source of Nanoparticles for Efficient Colloid thrusters, Journal of Propulsion and Power, Vol. 17, pp. 977-987 (2001), the subject matter of which is herein incorporated by reference in its entirety, reported that when using 20 micron tips and when operating with high conductivity, formamide solutions and low liquid flow rates injected into the meniscus, about half of their solvent was lost by evaporation rather than being ejected as drops.
Solvent volatility therefore introduces serious limitations, even in the case of solvents that can be electrosprayed in a low-pressure environment for the following reasons: the mass lost by evaporation does not produce thrust, and is therefore wasted from the viewpoint of space propulsion. The loss of solvent may lead to-salt precipitation and emitter clogging, and the avoiding of such a catastrophic instance requires the use of solvent concentrations well below saturation, which in turn limits the electrical conductivity of the solution and hence its performance in electrical propulsion. The present invention not only enables the formation of Taylor cones of liquids which would ordinarily boil or freeze, but also improves the performance of moderately volatile liquids included in the earlier invention, which neither boil nor freeze, but whose volatility limits their performance.
The maximum charge to mass ratio that electrolytes of glycerol or formamide, and ionic liquids are able to deliver as pure drops is limited by the onset of ion evaporation below a critical drop size, which reduces drastically the propulsion efficiency and introduces other complications. Their ability to operate in the pure ion evaporation mode is also limited at room temperature by the finite electrical conductivities of these substances. From the viewpoint of electrical propulsion and many others, it would be advantageous to be able to attain still higher charge to mass ratios within the pure drop regime, as well as lower charge to mass ratios within the pure ion regime.
Water is an exceptional solvent, with singular values of the electrical conductivity, surface tension and ion solvation energy, as well as stability with acids and bases. These properties would allow for the production of low-pressure sources of ions and drops that are far better than currently available materials. A number of volatile solvents other than water may also have considerable advantages.
The goal of the present invention is to enable the formation of Taylor cones of volatile liquids in a vacuum or in a low-pressure environment. The advantages of doing so, and the means to attain this goal are discussed below mainly in relation to the problem of electrical propulsion. However, similar applications to other fields making use of beams of ions and charged nanometer drops are equally evident, and are also considered as part of this invention. Although the illustrative case of water is mainly discussed as an example of a volatile liquid to be electrosprayed at low pressure, other volatile solvents are also of considerable interest, and are included as part of this invention. This invention is not limited either to the case of high conductivity liquids, but includes all volatile liquids that can be sprayed in a vacuum or a low-pressure environment by the proposed means.
The present invention is directed to a method of forming a stable electrospray of a volatile liquid in a low pressure environment comprising the steps of:
a) supplying one or more liquids into an emitter electrode located in a low pressure environment, wherein one of said liquids is a volatile liquid, and
b) establishing a voltage difference between said emitter electrode and one or more surrounding electrodes or grids, wherein a meniscus formed by said one or more liquids supplied to said emitter electrode forms one or more Taylor cones, from whose tip region at least drops and/or ions of said one or more liquids are ejected,
wherein a tendency of said volatile liquid to freeze, boil, or evaporate is diminished by a suitable evaporation-reduction means.
In a preferred embodiment, two liquids are supplied into the emitter electrode, a volatile liquid and an involatile liquid, and the evaporation-reduction means comprises covering most of the free surface of the volatile liquid with a layer of the involatile liquid, so as to minimize direct exposure of the volatile liquid with the low-pressure region.
In a second preferred embodiment, only the volatile liquid is supplied into the emitter electrode, and the evaporation-reduction means comprises reducing the dimensions of the free surface of the volatile liquid exposed to said low pressure region to below a critical value, such that evaporative cooling does not cause the Taylor cone to freeze.